materials Review Sputtered LiCoO2 Cathode Materials for All-Solid-State Thin-Film Lithium Microbatteries Christian M. Julien 1,* , Alain Mauger 1 and Obili M. Hussain 2 1 Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), CNRS UMR 7590, Campus Pierre et Marie Curie, Sorbonne Université, 4 place Jussieu, 75005 Paris, France 2 Thin Films Laboratory, Department of Physics, Sri Venkateswara University, Tirupati 517502, India * Correspondence: [email protected] Received: 20 July 2019; Accepted: 20 August 2019; Published: 22 August 2019 Abstract: This review article presents the literature survey on radio frequency (RF)-magnetron sputtered LiCoO2 thin films used as cathode materials in all-solid-state rechargeable lithium microbatteries. As the process parameters lead to a variety of texture and preferential orientation, the influence of the sputtering conditions on the deposition of LiCoO2 thin films are considered. The electrochemical performance is examined as a function of composition of the sputter Ar/O2 gas mixture, gas flow rate, pressure, nature of substrate, substrate temperature, deposition rate, and annealing temperature. The state-of-the-art of lithium microbatteries fabricated by the rf-sputtering method is also reported. Keywords: thin films; sputtering technique; lithium cobaltate; cathode material; lithium microbattery 1. Introduction Rechargeable thin-film lithium microbatteries (LMBs) have been developed to power wearable electronic microdevices as a noise-free power source [1]. All-solid-state LMBs have attracted more and more interest due to safety and chemical stability issues of the solid electrolyte. The first microbattery made at Oak Ridge National Labs (ORNL) was integrated to complementary metal oxide semiconductor (CMOS) chips as a memory for the basic input/output system (BIOS) of computers [2–4]. Currently, there is an increasing demand for thin-film microbatteries. While the global Li-ion battery market is expected to surpass $77 billion by 2024, the thin-film batteries market is forecasted to reach $1.72 billion [5]. Since the invention by the ORNL group, numerous studies have been devoted to the design and optimization of the best positive electrode material, which is the limiting element of a microbattery (i.e., the capacity delivered by the power source is governed by the material). In particular, LiCoO2 (LCO), which was identified as a promising candidate cathode material for Li-ion batteries (LIBs) [6], 1 yields a practical specific capacity of 135 mAh g− and it shows a fast charge-discharge reaction in the + potential range from ~3.8 V (fully lithiated state) to ~4.2 V vs. Li /Li (charge state at Li0.5CoO2)[7]. Among the various physical deposition techniques, radio-frequency (RF) magnetron sputtering is a powerful method to prepare oxide thin-films by controlling their crystalline structure and surface roughness with a composition similar to that of the target material. Features that make rf-magnetron sputtering a suitable technique for all-solid-state batteries can be summarized as follows. This deposition method can operate in a reactive atmosphere at low pressure (0.1 to 10 Pa). Using a high frequency of 13.56 MHz to adequately ionize the gas, there are reduced arcing and charge-up effects. Finally, due to the highly accelerated particles arriving on the substrate (10 to 40 eV), the rf-sputtered coatings have superior adhesion compared with other techniques. In addition, the chamber can include the multiple targets needed for successive deposits during the manufacture of a microbattery. Materials 2019, 12, 2687; doi:10.3390/ma12172687 www.mdpi.com/journal/materials Materials 2019, 12, 2687 2 of 26 Minerals 2019, 12, 2687 2 of 26 Various intercalation compounds such as thin-film cathodes for LMBs have been fabricated by Various intercalation compounds such as thin-film cathodes for LMBs have been fabricated by rf-magnetron sputtering, i.e., MoO3,V2O5, LiMn2O4, LiCoO2, LifePO4, WO3, etc. [8–11]. Actually, rf-magnetron sputtering, i.e., MoO3, V2O5, LiMn2O4, LiCoO2, LifePO4, WO3, etc. [8–11]. Actually, sputtered LiCoO2 films have become the most popular electrodes for thin-film microbatteries. The sputtered LiCoO2 films have become the most popular electrodes for thin-film microbatteries. The choice of this material comes from several reasons. (i) It has demonstrated high electrochemical choice of this material comes from several reasons. (i) It has demonstrated high electrochemical performance due to its lamellar structure favorable for Li+ ion pathway. (ii) It is a ternary compound performance due to its lamellar structure favorable for Li+ ion pathway. (ii) It is a ternary compound whose synthesis is more easily controllable than other compound performers like LiFePO4 (LFP) and whose synthesis is more easily controllable than other compound pe+ rformers like LiFePO4 (LFP) and LiNixMnyCo1-x-yO2 (NMC). (iii) It has a higher voltage (>4 V vs. Li /Li) and delivers higher energy LiNixMnyCo1-x-yO2 (NMC). (iii) It has a higher voltage (>4 V vs. Li+/Li) and delivers higher energy density than LiFePO4 olivine or LiMn2O4 spinel. (iv) The growth with preferential orientation for fast density than LiFePO4 olivine or LiMn2O4 spinel. (iv) The growth with preferential orientation for fast kinetics is relatively easy. kinetics is relatively easy. LiCoO2 (LCO) crystallizes in the rhombohedral structure (space group R-3m) with atoms in the LiCoO2 (LCO) crystallizes in the rhombohedral structure (space group R-3m) with atoms in the following Wykoff positions: Co in 3a (0,0,0), Li in 3b (0,0, 1 ) and O in 6c sites (0,0, 1 ). Li and Co atoms following Wykoff positions: Co in 3a (0,0,0), Li in 3b (0,0,½)2 and O in 6c sites (0,0,¼).4 Li and Co atoms are alternately located on octahedral sites between adjacent close-packed (ccp) planes of oxygen. The are alternately located on octahedral sites between adjacent close-packed (ccp) planes of oxygen. The unit cell of the LCO structures is presented in Figure1. The ordered structure is known as HT-LiCoO 2 unit cell of the LCO structures is presented in Figure 1. The ordered structure is known as HT-LiCoO2 due to its formation at a temperature higher than 600 C (Figure1a), while the low-temperature due to its formation at a temperature higher than 600◦ °C (Figure 1a), while the low-temperature modification (LT-LiCoO2) is slightly disordered with a cubic structure (space group Fd3m; Figure1b). modification (LT-LiCoO2) is slightly disordered with a cubic structure (space group Fd3m; Figure 1b). The existence of the HT-LiCoO2 structure is generally confirmed by a distinct splitting of the (110)/(108) The existence of the HT-LiCoO2 structure is generally confirmed by a distinct splitting of the doublet of the XRD diffraction lines. Note in the ideal ccp lattice the c/a ratio is 2p6 (4.899). For the (110)/(108) doublet of the XRD diffraction lines. Note in the ideal ccp lattice the c/a ratio is 2√6 (4.899). well-crystallized HT-LiCoO2 phase synthesized by the sol-gel via malic acid assisted method, the c/a For the well-crystallized HT-LiCoO2 phase synthesized by the sol-gel via malic acid assisted3+ method,6 0 ratio is reported to be 4.987 [12]. Magnetic properties indicated that LiCoO2 consists of Co (t2g eg ) the c/a ratio is reported to be 4.987 [12]. Magnetic properties indicated that LiCoO2 consists of Co3+ ions in the low-spin state [13]. (t2g6eg0) ions in the low-spin state [13]. Figure 1. View of the structure of LiCoO .(a) The high-temperature (HT) phase, rhombohedral Figure 1. View of the structure of LiCoO2 2. (a) The high-temperature (HT) phase, rhombohedral structure (space group R-3m). (b) The low-temperature (LT) phase, cubic structure (space group Fd3m). structure (space group R-3m). (b) The low-temperature (LT) phase, cubic structure (space group Reproduced with permission from [14]. Copyright 1995 Elsevier. Fd3m). Reproduced with permission from [14]. Copyright 1995 Elsevier. The investigation of electrochemical behavior of LixCoO2 by the cyclic voltammetry or The investigation of electrochemical behavior of LixCoO2 by the cyclic voltammetry or galvanostatic method in both non-aqueous (aprotic) and aqueous electrolytes demonstrated the phase galvanostatic method in both non-aqueous (aprotic) and aqueous electrolytes demonstrated the changes in the potential window 3.0–4.3 V due to the (de)intercalation reactions in the composition phase changes in the potential window 3.0–4.3 V due to the (de)intercalation reactions in the range 1.0 x 0.5 [15,16]. As shown in Figure2, a typical voltammogram of Li xCoO2 in organic composition≥ ≥range 1.0 ≥ x ≥ 0.5 [15,16]. As shown in Figure 2, a typical voltammogram of LixCoO2 in electrolyte exhibits three anodic/cathodic couples. The broad peaks at 4.08/3.83 V are associated with organic electrolyte exhibits three anodic/cathodic couples. The broad peaks at 4.08/3.83 V are the two-phase domain (0.75 x 0.95) and the two pairs of redox peaks at 4.13/4.03 and 4.21/4.14 V associated with the two-phase≥ ≥domain (0.75 ≥ x ≥ 0.95) and the two pairs of redox peaks at 4.13/4.03 are related to the interlayer Li vacancy in-plane ordering [17]. LiCoO2 in a saturated Li2SO4 aqueous and 4.21/4.14 V are related to the interlayer Li vacancy in-plane ordering [17]. LiCoO2 in a saturated solution demonstrated similar electrochemical features [18]. Li2SO4 aqueous solution demonstrated similar electrochemical features [18]. Well-crystallized LCO films exhibit a clear discharge voltage plateau at ca. 4 V vs. Li+/Li, but the discharge profile appears sloping for amorphous-like films. Most of the commercial all solid-state thin-film lithium microbatteries constructed with a LCO cathode, a fast Li+-ion conductor as the solid 2 1 electrolyte, and a metallic Li anode, achieve initial specific capacities of ~60 mAh cm− µm− on Materials 2019, 12, 2687 3 of 26 2 1 2 1 discharge, which is close to the theoretical value ~68.9 µAh cm− µm− or 248 mC cm− µm− (i.e., 1 3 correspondingMinerals 2019, 12, to2687 gravimetric capacity of 137 mAh g− for a density of 5.06 g cm− and Li uptake x3 =of0.5 26 in LixCoO2)[19].