Stress Evolution of Thin Film Ruo2 Li-Ion Battery Electrodes by Brian

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Stress Evolution of Thin Film RuO2 Li-ion Battery Electrodes

Brian Mills

Submitted to the Department of Physics in partial fulfillment of the requirements for the degree of

Bachelor of Science in Physics

at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

May 2020

Author ...... Brian Mills Department of Physics May 8, 2020

Certified by...... Carl Thompson Professor of Material Science and Engineering Thesis Supervisor

Certified by...... Joseph Checkelsky Associate Professor of Physics Thesis Supervisor

Accepted by ...... Nergis Mavalvala Associate Department Head, Department of Physics

Stress Evolution of Thin Film RuO2 Li-ion Battery Electrodes

Brian Mills

Submitted to the Department of Physics on May 8, 2020 in partial fulﬁllment of the requirements for the degree of Bachelor of Science in Physics

Abstract Thin Film Li-ion batteries (TFB) are seen as a promising candidate for powering small, low power microelectronic devices as they exhibit high energy density and can operate reliably at low voltages. Currently the biggest obstacle to TFB battery development is high volume expansion and material degradation in electrodes with high theoretical Li ion capacities. Among these materials is RuO2, which exhibits excellent capacity and great potential for use as either cathode or anode in low power electronics. In order to better understand the mechanisms that underlie mechanical failure in RuO2, we perform the ﬁrst in situ measurement of mechanical stress evolution in thin ﬁlm RuO2 electrodes. The results of these measurement reveal a very unique stress evolution pattern in RuO2, which has not been observed or modeled in any previous experiment, exhibiting near zero stress delithiation and linear increase in stress during lithiation. These results point to a mode of failure of RuO2 which does not occur in other materials currently being studied.

Thesis Supervisor: Carl Thompson Title: Professor of Material Science and Engineering

Thesis Supervisor: Joseph Checkelsky Title: Associate Professor of Physics

3 Contents

1 Introduction5

1.1 Thin Film Battery Materials...... 6 1.2 Ruthenium Oxide Electrodes...... 9

2 Stress in Thin Films 11

2.1 Introduction...... 11 2.2 Stress-Strain Curve...... 15 2.3 Thin Film Curvature...... 17 2.4 In Situ Stress Measurements...... 20

3 Experimental Setup 22

3.1 Sample Fabrication...... 22 3.2 Cell Construction and Cycling...... 23

4 Results and Discussion 25

4.1 Stress and Electrochemical Measurements...... 25 4.2 Discussion...... 29 4.2.1 Crack Closure...... 30 4.2.2 Conversion Reaction Film Growth...... 32 4.2.3 General Remarks...... 32

5 Summary and Future Work 33

4 1 Introduction

Li-ion batteries (LIB) have seen extensive use in the production of modern day electronics. Their high power density and capacity, as well as reliable cycling has made them the ideal choice for secondary (ie., rechargeable) batteries in many applications. Advances in LIB design have been necessary in accommodating the development of more powerful and smaller electronic devices [1]. The sustained movement towards internet of things (IoT) technology has further increased demand for highly efficient, reliable and compact power supplies, especially for systems with low operating voltages and currents. An exceedingly promising candidate for these types of applications is the thin film Li-ion battery (TFB), which has shown considerably larger gravimetric (per unit mass) and volumetric (per unit volume) capacities than other battery designs [2]. TFBs are also entirely built from solid state components, which addresses long- standing safety concerns with the volatility of conventional liquid electrolyte batteries. Fabrication of TFBs generally consists of sequentially depositing 100 nm-1 µm thick layers of various materials using film deposition techniques such as DC and RF sputtering. Sometimes conditioning sequences, such as thermal annealing, are conducted between these depositions to optimize performance of certain materials [3]. Figure 1.1 shows the layout of a typical TFB.

Figure 1.1: A schematic of a typical layering of materials used to construct a TFB.

5 1.1 Thin Film Battery Materials

Advances in TFBs have largely been driven by breakthroughs in new materials and fabrication methods. There are significant benefits to the development of TFBs in which all components are compatible with complementary metal-oxide-semiconductor (CMOS) fabrication methods that are the standard in manufacture of modern microelectronics. Such an advancement would allow for microchips with completely integrated power supplies, constituting a large increase in the efficiency of manufacturing as a whole.

Many of the currently commercialized TFBs for integrated circuits follow a standard design: lithium metal-oxide cathode, solid electrolyte, and lithium metal anode. The electrode materials are reactants in an oxidation reduction reaction, requiring the exchange of Li ions and electrons between the cathode and anode. The electrolyte allows for movement of Li ions between the electrodes while also electrically isolating them. In order to complete the reaction, electrons ﬂow through an external circuit that connects the electrodes, generating current. Lithium metal-oxides currently in use as cathode materials, such as LiCoO2 and LiMn2O4, have theoretical −1 gravimetric capacities of 150-300 mA h g and theoretical volumetric capacities that −3 −3 exceed 1000 mA h cm for some species, with ~500 mA h cm being successfully harnessed in commercialized designs. The solid electrolyte that is almost exclusively used in TFBs is lithium phosphorus oxynitride — LiPON. LiPON is a glassy, amor- phous solid and is appealing as an electrolyte because of its relatively good ionic conductivity and mechanical stability it provides to battery structure. It is this added stability from LiPON which allows Li metal to be used as the anode in TFBs, as it is found that it suppresses dendrite growth in the cell which lead to electrical shorts

6 between the electrodes – a leading cause of exploding batteries[4]. In bulk batteries where Li metal cannot reliably be used, graphite is the preferred material, and as a result has also been extensively studied for use in TFBs. It has theoretical gravi- −1 −3 metric and volumetric capacities of 339 mA h g and 760 mA h cm respectively and is used as the anode material in many TFBs. The main appeal of graphite is the stability of its crystalline structure during charge and discharge, which results in reliable cycling and a longer lifetime. The common thread between the current materials of choice for TFB—and a large portion of Li-ion batteries in general—is that they incorporate Li ions through intercalation. In intercalation materials, Li ions are incorporated into a material’s existing crystallographic structure, for example between layers of LiCoO2, and can be reversibly extracted without drastically altering the surrounding material structure. These intercalation type materials have been the focus of extensive research, which has shown the deep relationships between crystal structure, insertion mechanisms, Li ion capacity and reaction reversibility [5].

Extensive research has also be conducted to assess and increase the viability of alloying materials for use as both positive and negative electrodes in Li ion batteries. As opposed to intercalation materials, these alloying materials have signiﬁcant structure changes when they react with Li ions, but tend to also have much higher theoretical capacities. Group IV elements such as Si and Ge that form alloys with Li, have −3 low potential vs Li, and can have theoretical capacities as high as 3000 mA h cm for some elements. However, disruption of crystal structure in these materials generally results in extremely high volume change during lithiation (addition of Li) and deliathion (removal of Li)—over 240% for Si, Ge and Sn, compared to only 10% in graphite [1][2][6]. High volume expansion leads to pulverization of ﬁlms and loss

7 Figure 1.2: Capacity trends for different types of electrode materials for Li-ion batteries. of electrical contact between regions of the electrode and current collector and thus irreversibly reduces battery capacity. For these reasons there has been considerable effort in creating alloying materials that maximize structural integrity while main- taining high specific capacity. This is often done through a combination of making composite materials, such as Mg2Si or Si-C composites, and creating nanostructures within the electrode material to accommodate the large volume changes [7][8].

While experimentation with new materials has yielded some improvement in cycling performance of TFB, the exact mechanisms through which mechanical stress in thin ﬁlm electrodes evolves during lithiation and delithiation, and how that stress may be alleviated, is not well understood. Establishing the link between electrode volume expansion and internal ﬁlm stress which drives degradation of the electrode is crucial to the development of TFB electrodes with reliable cycling performance. Study of these

8 phenomena in intercalation and alloying materials has yielded insightful results that deepen our understanding of how they operate within a TFB cell [9][10][11]. This motivates a continuation of these analyses for conversion materials. A conversion material is one in which incorporation of Li ions entails the replacement of the existing crystal matrix at the electrode with one of significantly different structure and chemical properties. Like alloying materials, they often feature significantly higher capacities than intercalation materials, but also suffer from high volume expansion and irreversible capacity loss.

1.2 Ruthenium Oxide Electrodes

RuO2 is a common material used in the research of supercapacitors [12][13] but has recently seen some application as a Li-ion battery electrode [14][15]. With 2.2 V potential vs Li, RuO2 electrodes can feasibly be used as either cathode or anode materials in low power applications in many types of microelectronics [16]. Mea- −1 sured first cycle specific capacities for RuO2 are reported to be >1000 mA h g and −3 ~500 mA h cm [17], with conversion reaction and interfacial storage between RuO2 and converted Ru metal nanocrystals accounting for most of the capacity [18]. RuO2 is also compatible the complementary metal-oxide-semiconductor (CMOS) fabrication techniques which are the standard in microelectronics fabrication industry. This is a hugely beneficial property for an electrode material, as an entirely CMOS compatible TFB would greatly streamline the manufacturing process by eliminating fabrication steps that would be necessary for certain electrode materials. However, irreversible capacity loss due to volume expansion (~100% in RuO2) has been a major obstacle in creating viable RuO2 electrodes for TFBs. This thesis covers how to con-

9 duct in situ stress measurements of RuO2 ﬁlms during battery cycling, and analyzes the unique stress evolution that RuO2 exhibits.

10 2 Stress in Thin Films

2.1 Introduction

In a broad sense, stress and strain are generalizations of the concepts of force and displacement that allow us to describe how non-rigid bodies deform under the application of forces. In the simplest case, forces acting along the axis of a rod L0 which stretches the rod by some amount ΔL is described in terms of the strain and stress in the rod. The stretching of the rod behaves similarly to a spring and follows Hooke’s law.

F = kΔL

Figure 2.1: Sketch of simple deformation of a rod along its axis described by Hooke’s law.

By normalizing this equation with respect to the cross sectional area of the rod we arrive at the stress strain relation for 1 dimension

= E" (2.1)

F ΔL ≡ " ≡ where the stress, A , is the force per unit area and strain, L0 is the fractional change in the length of the rod. E is the Young’s modulus, which is analogous to the spring constant. Materials that exhibit this linear relationship between stress and

11 strain are called linear-elastic materials.

In order to describe deformations caused by arbitrary forces acting on an object, the stress and strain are decomposed into components that are perpendicular and parallel to the surface they act on, resulting in the tensor equation:

ij = Cijkl"kl (2.2) which is known as generalized Hooke’s law, where ij is the stress tensor, "ij is the strain tensor and Cijkl is the stiﬀness tensor. We use Einstein notation to denote summing over indices. Indices i and j represent, respectively, the axis along which the force acts and the direction of the normal vector of the surface on which it is acting. Similarly k and l describe the directions of the deformations with respect to the surface normal vector.

(a) A normal stress. (b) A shear stress.

Normal stresses and strains are given by diagonal components ii and "kk respectively, and shear stresses and strains are described by other components. What was simply the Young’s modulus in Eq 2.1 has become a more complicated rank 4 stiﬀness tensor

Cijkl. This is because in general a stress in a given direction may cause strain in

12 many diﬀerent directions. Applying this to our original 1-D example: 11 = E"11,

so C1111 = E. It is also common to see the generalized Hooke’s law written in inverse form as

"ij = Sijklkl (2.3) where the compliance tensor Sijkl is the inverse of Cijkl.

Figure 2.3: Sketch of the transverse response to axial stress on an object.

As mentioned before, a single stress may cause a variety of strains. For example, as shown in Fig 2.3, an axial stress on an object may stretch it along that axis but contract

it along perpendicular axes. The ratio between the axial strain "11 and transverse strains "22 and "33 is known as the Poisson ratio, and is represented by the symbol " = " = −" = − . For an isotropic material, the resulting strains are 22 33 11 E 11. The negative is because in general we expect a material that is being stretched in one direction to contract in others; although there are special materials that violate this. It

13 can also be shown that the shear moduli, which are components of Sijkl where i = k j = l 1+ and , can be expressed as E in isotropic materials. The compliance tensor that describes this system under an arbitrary stress is then given by

⎧ ⎪ 1 , i = j = k = l ⎪E ⎪− , i = j, k = l, i k ⎪ E ≠ Sijkl = ⎨1+ ⎪ , i = k, j = l, i ≠ j ⎪ E ⎪ ⎪0 Otherwise ⎩ which when used in Hooke’s law yields the constitutive equations

1 " = ( (1 + ) − ) ij E ij ij kk (2.4)

The energy stored within the material as a result of elastic deformations can also be described using the stress and strain components. Starting from the usual energy in terms of the force and displacement of the object we write an expression for energy density u for a simple one dimensional case.

dU dU = F dx → = d" dV

u = d" (2.5) Ê

For a linear elastic material this becomes

1 u = E"2 2

To generalize to our strain tensor we simply sum the contribution to the energy of

14 each component 1 1 u = " = C " " 2 ij ij 2 ijkl ij kl (2.6)

2.2 Stress-Strain Curve

So far we have focused on systems that exhibit linear elasticity, ie. systems that follow Hooke’s law Eq 2.2. If we were to plot the stress vs strain on an object as it is deformed, this curve would be linear, with the elastic modulus deﬁning the slope. However, if an object is stressed passed a certain point, called the yield stress, the relationship between the stress and strain in the object is no longer linear. Deforma- tions in this regime of non-linear stress and strain are referred to as plastic deformations. Behavior of a material in this plastic regime is more complex than in the elastic regime, but forms the basis from which properties such as brittleness and ductility can be understood. The way in which a material deforms in the plastic regime is called plastic ﬂow.

An important property that is often exhibited by materials that have been plastically deformed is elastic unloading. This means that once the external force causing the strain is released, the object relaxes by following a linear stress-strain curve. Typi- cally this means that plastically deforming a material then allowing it to relax will result in an unstressed length that is diﬀerent form the original. A typical stress strain curve is shown in Fig 2.4 with labeled linear elastic and plastic regions, as well as linear unloading.

For thin film batteries, the stress vs capacity curve is synonymous with the stress- strain curve of the film. This is because in general the Li content in a film is directly

15 proportional to the amount of volume expansion, or bulk strain, it is subject to. This proportionality has been shown experimentally in the case of Si ﬁlms [19]. Electrodes are compressed ( < 0) during lithiation and are under tensile stress ( > 0) during deliathion. The stress capacity curve in a battery electrode during electrochemical cycling typically follows a pattern of

Elastic Unloading → Elastic Loading → Plastic Flow (Delithiation)

→ Elastic Unloading → Elastic Loading → Plastic Flow (Lithiation)

creating a hysteresis loop like the sketch shown in Fig 2.5.

Yield Point Plastic Flow

Elastic Loading

Elastic Unloading

0 ε

Figure 2.4: Typical stress-strain curve of an object that is plastically deformed, then unloads linearly.

16 Figure 2.5: Taken from Sethuraman et. al. [10] with permission. Shows the stress vs capacity curve of a Si ﬁlm during lithiation and delithiation.

2.3 Thin Film Curvature

Thin films can be modeled as isotropic biaxial systems in which there is no out-of- plane or shear components of stress or strain. The isotropy of the film is a reason- able assumption for sputtered nanocrystalline films which do not show significant anisotropy for longer length scales. Assuming isotropic biaxial system means the

only non-zero components of stress and strain are 11, 22, "11and "22. Using Eq 2.4 we write the stress strain relations

1 1 " = ( − ) " = ( − ) 11 E 11 22 22 E 22 11

which can be further simpliﬁed if we assume 11 = 22 = B, yielding

" = " = B 11 22 M (2.7)

17 biaxial modulus M = E where the 1− is the relevant constant of proportionality.

It is well established that thin films deposited on substrates exhibit naturally occuring residual stresses [20][21]. These stresses are often the result of mismatch between substrate and film material properties, such as lattice constant and thermal expansion coefficient. It is also well known that stress within a thin film on a substrate induces a curvature in the substrate, described by the Stoney equation [22]. In this section we use a similar method to that of Freund and Suresh [23] to derive the Stoney equation.

In order to conceptualize how ﬁlm mismatch stresses result in substrate curvature, we

think of a L0xL0xℎs substrate and L0xL0xℎf film. Imagine holding the film above the substrate, while externally applying a force f which is equal to all mismatch stresses that would be associated with being constrained to the substrate. Then the film is connected to the substrate and the external force is removed; as the film at- tempts to relax to a stress free state it exerts a force on the substrate resulting in curvature. This is visualized in Fig 2.6a.

(a) Film stress causes substrate curvature (b) Strain resulting from curvature of substrate

Figure 2.6: Relationship between thin ﬁlm stress and substrate curvature/strain.

This curvature can be related to the strain in the substrate. For an isotropic material

18 with only small deformations, we assume the curvature is spherical and is given by = 1 R. Using Fig 2.6b as a reference, we see that during bending the mid plane of the substrate does not change length, but every plane above or below the mid plane has a diﬀerent length, which is the source of the strain. If the substrate subtends angle , we can write the strain as

L − L0 (R − z) − R z "11(z) = = = − = −z L0 R R

By similar reasoning we also have "22 = −z. Using Eqs 2.6 and 2.7 the strain energy density as a result of this curvature is

2 2 uc = M z

We integrate over the volume of the substrate to get the total strain energy.

2 2 L ℎs∕2 ML U = u dV = 0 2 z2dz = 0 2ℎ3 c c M 12 s (2.8) Ê Ê−ℎs∕2

The strain energy from the force f which acts in the opposite direction on the substrate z = ℎ at 2 is given by ℎ U = −f"( )L = −L ℎ f f 2 0 0 s (2.9)

Combining Eqs 2.8 and 2.9 gives the total strain energy in the substrate ML2 U = U + U = 0 2ℎ3 − L ℎ f c f 12 s 0 s

which can be minimized with respect to to give

6f = 2 L0Mℎs

19 f f = f ℎf Finally we note that L0 is related to the ﬁlm stress and thickness via L0 . This gives the usual form of the Stoney equation [22].

6 = ℎ 2 f f (2.10) Mℎs

It is important to note the assumptions of this model, which govern the regime in which the Stoney equation is accurate:

• Uniform pure biaxial stress (33=0) and isotropic substrate.

• Small deformations which allows us to assume linear elasticity.

– Implicit in this is that ℎs >> 1 and ℎs >> ℎf .

• Constant curvature throughout substrate, which ignores edge eﬀects.

2.4 In Situ Stress Measurements

The Stoney model allows for measurement of thin film stress-thickness f ℎf through direct measurements of substrate curvature. This has been done to measure residual film stress, as well as temperature due to thermal expansion and film growth [24][25][26]. Similar stress evolution studies have also been conducted to charac- terize mechanical behavior of thin film Si and Ge during lithiation and delithiation [10][11]. In situ measurements of thin film stress are often done through reflecting a laser off the surface of the substrate and measuring the deflection of the laser as a result of curvature. A particular implementation of this type of measurement scheme is a multi beam optical sensor (MOS) [27][28].

20 As shown in Fig 2.7, the sensor consists of a collimated laser which is passed through an etalon to create multiple parallel beams. After being reflected off a sample, the beam is captured using a CCD camera. The beam spacing after reflecting off a substrate can be used to measure the local curvature.

Figure 2.7: Schematic of a multi beam optical sensor.

The relation between sample curvature and beam spacing measured by the CCD camera is given by Δd Δ = Am d0 where Δ is a small change in curvature of the substrate, Δd is a small change in beam spacing and d0 is a reference beam spacing. Calibration of the system using A = cos mirrors of known curvature can be used to calculate a mirror constant m 2L that allows for precise conversion between beam spacing measured by the CCD and substrate curvature for a particular setup.

21 3 Experimental Setup

In situ measurement of stress in a RuO2 thin ﬁlm during lithiation and delithiation is conducted using a MOS setup and specialized beaker cells. Battery half cells are created using a thin ﬁlm active electrode and Li metal counter electrode with a liquid electrolyte. The curvature of the substrate of active electrode is measured as the cell is cycled at constant current.

3.1 Sample Fabrication

(a) (b)

(c)

Figure 3.1: Schematic of electrode samples. (a): Top view of the sample; acted as the reﬂective surface for use in MOS stress measurement. (b): Bottom view of the sample; contained the active electrode and current collector for electrochemical cycling. (c): Cross sectional view of sample with thickness of each layer. Figures not drawn to scale.

Fig 3.1 shows a schematic of the electrode sample. The electrode sample was fabri- cated using sputter deposition on a 1” diameter, ~500 µm thick fused silica substrate. To ensure high purity and strong adhesion of ﬁlms, the substrates are cleaned with ar- gon and oxygen plasma prior to each deposition. Sputter deposition allows for dense,

22 uniform layers of materials to be layered with strong adhesion, making it the ideal choice of fabrication method for the materials used in the electrode. A 100nm Ti layer was deposited on both sides of the substrate using DC sputtering. The layer on the "top" side of the substrate (Fig 3.1a) serves as the reﬂective surface that allows for substrate curvature measurements using a MOS setup. On the "bottom" side (Fig 3.1b), Ti acts as an adhesion layer to limit delamination of deposited materials from the substrate. A 100 nm Al layer is then deposited as a current collector of the active electrode in the battery half-cell. Using Al as a current collector ensures uniform distribution of current to the entire electrode. The ductility of Al also lessens the likelihood of crack propagation into layers below the RuO2 electrode, keeping the entire electrode electrically connected even as the active material separates into is- lands. The active electrode is a 200 nm RuO2 ﬁlm deposited on the Al layer using RF sputtering. RF sputtering is used to reduce the build up of charged particles at the RuO2 target and enhance deposition rate. RuO2 is deposited using O2 processing −1 gas at 3 mTorr working pressure, which results in a deposition rate of ~0.2 Å s .A

2 cm diameter shadow mask is used to control the radius of the RuO2 ﬁlm and leave an exposed region of Al to which an electrical connection can be made.

3.2 Cell Construction and Cycling

The battery half cell is constructed in a beaker cell, using a lithium metal counter electrode and liquid electrolyte LP30 (1 M LPF_6 in a 1:1 volume ratio mixture of ethy- lene carbonate and dimethyl carbonate). A separator is placed between the working and counter electrodes to prevent a short circuit between the electrodes. The working electrode is free-standing, as opposed to compressing it with a spring as would be

23 common in other cells, in order to prevent any external forces that may cause additional substrate curvature. The cell consists of an optical window which allows for MOS measurements, and is tightly sealed to prevent air from entering and reacting with sensitive components like the lithium metal and electrolyte. A schematic of the cell as is used in the MOS apparatus is shown in Fig 3.2.

Figure 3.2: Schematic of assembled beaker cell.

The cell was placed in the MOS apparatus and connected to a potentiostat to be cycled. The cells exhibited open circuit voltage of ~3.5 V and were cycled at constant current of 68 µA which is approximately C/5 rate based on the theoretical speciﬁc capacity of the RuO2. C/5 is chosen because at this rate RuO2 maintains high capacity and shows no signiﬁcant overpotentials [16].

24 4 Results and Discussion

4.1 Stress and Electrochemical Measurements

Here we present and discuss the results obtained from cycling RuO2 electrodes at constant C/5 rate. Using the Stoney equation (Eq 2.10) and the established value

of the biaxial modulus of fused silica (M = 87.5 GPa), the stress-thickness f ℎf calculated from curvature measurements, and is reported in units of GPa⋅µm. The stress-thickness is indicative of the ﬁlm stress, but does not assume constant thickness of the electrode ﬁlm, which may change during cycling. The established value for the biaxial modulus of the fused silica, M = 87.5 GPa, is used in these calculations. Capacity is obtained by integrating the current over time, which is then normalized to the volume of the sample. Fig 4.1 shows the stress-thickness measured during the

ﬁrst lithiation and delithiation of a RuO2 electrode sample.

0 Delithiation

00 m] μ

0 [GPa*

Lithiation

-Thickness 0

0 00 00 00 00 00 μ μ

Figure 4.1: Stress-thickness during the initial lithiation and delithiation of a RuO2 electrode. Arrows show direction that corresponds to lithiation and delithiation. The dotted line corresponds to Stress-Thickness=0.

m] μ

-Thickness [GPa* -Thickness

0 100 200 300 400 500 Volumetric μ μ

Figure 4.2: Second lithiation (green) shows linear increase in stress and ﬁlm is lithiated.

The initial lithiation and delithiation of the RuO2 film shows regions which can be identified as elastic and plastic deformations of the film. During the initial lithiation −2 −1 of the film, from about 0 to ~25 µA h cm µm the response is quite linear, followed by a non-linear region consistent with plastic deformation of the film past the yield −2 −1 stress. A short interruption of electrochemical cycling around 80 µA h cm µm allowed the film to relax, which is the source of the peak seen at this point. On resuming cycling, the film once again underwent a short period of elastic loading followed by non-linear plastic flow. The initial delithiation features a linear region towards 0 Gpa⋅µm stress-thickness, which shows linear unloading of the film. This is followed by a peak, and then sharp decline to near 0 Gpa⋅µm stress-thickness. The second lithiation sequence of the electrode exhibits a linear increase in compressive stress as the electrode is lithiated again, as shown by the green line in Fig 4.2. The near linear relationship between stress and capacity during initial lithiation of the RuO2

26 is maintained throughout subsequent cycles (Fig 4.3), although deviations are seen further into the lithiation process. Another important aspect of subsequent cycles is the lack of a peak during delithiation, which means after linear unloading, the RuO2 continues to delithiate close to 0 GPa⋅µm, a property that has not been previously measured for alloying electrode materials, such as Si (Fig 2.5).

Cycle 1 Cycle 2 Cycle 3

m] μ

-Thickness [GPa* -Thickness

0 100 200 300 400 500 600 Volumetric μ μ

Figure 4.3: First 3 cycles of a RuO2 electrode. General shape of hysteresis is consistent throughout all cycles.

The electrochemical data recorded during cycling, shown in Fig 4.4 is generally consistent with that of previous studies [16][15][14], further validating the stress measurements. The interruption of cycling and relaxation of the electrode is also reflected −2 −1 in voltage curves as the kink at ~80 µA h cm µm . In the measured cell capacity increased with subsequent cycles which, while not seen in usual circumstances, ap- pears to have no significant effect on the features of the stress evolution.

27 Cycle 1 3.5 Cycle 2 Cycle 3 3.0

2.5

2.0 Voltage [V] Voltage 1.5

1.0

0.5

0 100 200 300 400 500 600 Volumetric Capacity [ μ ] 2μ

Figure 4.4: Voltage hysteresis of a RuO2 electrode.

These stress evolution results were reproduced after cycling a different RuO2 electrode sample; stress and voltage hystereses are shown in Fig 4.5. Unfortunately due to issues with alignment of the MOS apparatus during curvature measurements of this sample, stress-thickness during early stages of cycling were incomplete. However, when compared to the previous sample data in Fig 4.3, we see reproduction of the key features exhibited by the electrode after the first delithiation. That is, this sample exhibited both delithiation near 0 GPa⋅µm after elastic unloading and gradual buildup of compressive stress during lithiation. Furthermore, changes in capacity between cycles are much smaller in this sample, yet the stress evolution profile is maintained. This strengthens the previously stated notion that large changes in measured capacity in the first sample are not related to stress evolution.

28 m] 1 μ

[GPa* 1

Second De-Lithiation Cycle 3 -Thickness Cycle 4

2 Voltage [V] Voltage 1

100 200 300 400 500 600 700 Volumetric Capacity [ μ ] 2μ

Figure 4.5: Voltage and Stress measurements of a second RuO2 electrode.

4.2 Discussion

Results obtained from measuring substrate curvature of RuO2 electrodes during electrochemical cycling are unlike the stress evolution of any other material for which these types of measurements have been conducted. For cases of Si and Ge which have been studied [10][11], the stress evolution profile is consistent with the model outlined in Section 2.2, generating considerable amounts of both tensile and compressive stress at different points during the cycle. RuO2 on the other hand generates only a modest tensile stress during the first cycle and no tensile stress in subsequent cycles. It is also important to note that while the initial lithiation shows a linear increase in compressive stress, this is not indicative of an elastic process. If a material is elasti- cally loaded without significant plastic deformation, it is expected that the relaxation of the material will be elastic, and therefore follow the same line as it relaxes. In this aspect as well, RuO2 differs from other studied materials, in which elastic unloading

29 is seen at the beginning of both lithiation and delithiation.

Given the reproducibility of the stress measurement proﬁle across multiple samples, we are conﬁdent that these measurements point to a unique mechanism of stress evolution in RuO2 which has yet to be explored. In this section we discuss mechanisms

which could potentially explain some of the features the seen in the cycling of RuO2.

4.2.1 Crack Closure

First, we consider a mechanism in which the RuO2 ﬁlm relieves stress through cracking and delamination. Detailed analytical descriptions of these types of fracture are given by Hutchinsion and Suo [29]. Fig 4.6 illustrates how this mode of failure may

allow reduction of stress in RuO2 electrodes. Channel cracks develop during the tensile portion of the ﬁrst delithiation of the ﬁlm. This is followed by delamination at

the interface between the RuO2 and Al layers. In this scenario, stress can be relieved through bending of the ﬁlm at edges where it has been disconnected.

(a) (b)

Figure 4.6: Schematic of channel cracking and delamination in a thin ﬁlm. (a) & (c) show channel cracks, (b) & (d) show delamination.

This model ﬁts the observation of a ﬁrst cycle peak in tensile stress followed by sub-

30 sequent cycles showing no tensile stress. This is because the initial peak would cor- respond to breakage of bonds at the interface between RuO2 and Al layers, and subsequent cycles would show no stress because the ﬁlm is not completely constrained to the substrate, and will itself bend at crack edges instead of imparting curvature to the substrate. It has also been shown that formation of channel cracks occurs in

RuO2 films that have been electrochemically cycled. An example of an SEM image of a 200nm RuO2 film after many cycles is shown in Fig 4.7. Within this model, however, if tensile stress reduction is the result of bending crack edges, one would also expect the film to exhibit no compressive stress during initial stages of lithiation, as the edges of cracked regions would bend downward to relieve compressive stress.

Observations of compressive stress immediately after lithiation of RuO2 has begun contradicts this.

Figure 4.7: SEM image of a 200nm RuO2 ﬁlm after many cycles. Wide channel cracks are observed throughout the ﬁlm [30]

31 4.2.2 Conversion Reaction Film Growth

Another factor which makes RuO2 distinct from other materials for which stress evolution has been analyzed is its incorporation of Li ions through a conversion reaction.

As opposed to intercalation or alloying, conversion reactions in RuO2 result in com-

plete replacement of regions of the original ﬁlm with a matrix of Li2O[31]. It may be

the case that differences in stress evolution between RuO2 and other materials is due to fundamental differences in how conversion reactions affect film stress. For exam-

ple, conversion from RuO2 to Li2O at the surface of the film may not result isotropic volumetric strain that is directly proportional to its Li content. Thus it breaks down an underlying assumption of our model that cell capacity is a representative measure for the strain in the film. Even in this framework, however, it still seems difficult to reconcile the apparent elastic unloading during delithiation but lack thereof in lithiation. Regardless, without a more thorough understanding of conversion reactions in thin film batteries and their effects on stress as well as thin film structure, no meaningful conclusions can be drawn.

4.2.3 General Remarks

The shortcomings of the models discussed in this section ultimately boil down to an

incompatibility with a deﬁning quality of the stress evolution of RuO2: a striking asymmetry with regards to lithiation and delithiation. As mentioned many times

to before, the RuO2 films generate substrate curvature when lithium is added, but not when it is removed. Explaining this will require a model which recreates the significant difference in behavior during lithiation and delithiation as well as properly describes the mode of failure which presumably occurs during the first delithiation process.

32 5 Summary and Future Work

Thin ﬁlm Li ion batteries are a promising candidate for the power source for low power applications such as passive sensing devices used in IoT technology. A key aspect in the future development of TFB to meet increasing demand for high density, reliable power supplies will be a deepening of our understanding of underlying mechanical processes that take place with TFB are cycled. In this work we investi- gate the stress evolution during electrochemical cycling of RuO2 electrodes for the

first time. RuO2 presents a unique stress evolution profile, characterized by near zero stress delithiation and linear plastic deformation during lithiation as well as a distinct peak seen only during the first delithiation. These results diverge greatly from materials for which this type of stress analysis has been done, but are reproduced across multiple samples. We present plausible mechanisms through which such stress evolution may arise, although it is difficult to fully reconcile all of our observations within a single model.

Future experimentation is necessary to identify the mechanisms that underlie the ob-

served stress evolution of RuO2. The largest gaps in our current understanding of this phenomenon are the details of the state of the ﬁlm at each point during cycling. Therefore a logical way to expand on this work is to take both surface and cross sectional images of the RuO2 at diﬀerent points during the cycle using an SEM. This will greatly narrow down the range of possible mechanisms and give important insight into how they may be probed. Another way to gain additional insight is to conduct stress measurements of a RuO2 sample with an additional layer of LiPON deposited on top. LiPON is well known to be a good stabilizing material in TFBs, and one

33 might expect that the additional stability could delay, or entirely inhibit, the mode of failure of the RuO2 seen in this experiment. This type of test would give good insight into whether it is the particular mode of failure or other aspects of RuO2, such as conversion reactions, that are the source of the stress evolution proﬁle’s unique features.

Overall, the results we present are a signal of some of the underlying complexities associated with broader ideas like cycling performance and reliability in TFB designs. Development and optimization of future TFB designs will rely on a more fundamental understanding of these complex processes to help guide our approach to maximizing their potential.

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