energies Review Current Li-Ion Battery Technologies in Electric Vehicles and Opportunities for Advancements Yu Miao 1,* , Patrick Hynan 2, Annette von Jouanne 2 and Alexandre Yokochi 1 1 Department of Mechanical Engineering, Baylor University, Waco, TX 76798, USA; [email protected] 2 Department of Electrical and Computer Engineering, Baylor University, Waco, TX 76798, USA; [email protected] (P.H.); [email protected] (A.v.J.) * Correspondence: [email protected]; Tel.: +1-254-710-2216 Received: 20 February 2019; Accepted: 16 March 2019; Published: 20 March 2019 Abstract: Over the past several decades, the number of electric vehicles (EVs) has continued to increase. Projections estimate that worldwide, more than 125 million EVs will be on the road by 2030. At the heart of these advanced vehicles is the lithium-ion (Li-ion) battery which provides the required energy storage. This paper presents and compares key components of Li-ion batteries and describes associated battery management systems, as well as approaches to improve the overall battery efficiency, capacity, and lifespan. Material and thermal characteristics are identified as critical to battery performance. The positive and negative electrode materials, electrolytes and the physical implementation of Li-ion batteries are discussed. In addition, current research on novel high energy density batteries is presented, as well as opportunities to repurpose and recycle the batteries. Keywords: Li-ion batteries; Li-ion battery chemistry; battery second-use; battery recycling; grid stabilization; Li-ion battery improvements 1. Introduction Electric vehicles (EVs) were first demonstrated in 1828 [1,2] with the first production electric car introduced in 1884 [3]. These EVs had clear advantages over the competing steam- and gasoline-powered vehicles, such as absence of the loud noise from an un-muffled internal combustion engine, and the difficult starting procedures that in early vehicles required the involvement of specialized staff that would initially heat those engines to operating temperature (the so-called “chauffeur”—i.e., “warmer”) [4]. The inventions of the internal combustion engine muffler is 1897 [5], the electric engine starter in 1911 [6], and the desire for larger vehicle autonomy and faster vehicle re-charging procedures all contributed to internal combustion engine powered vehicles eclipsing the use of EVs except in specialized uses. Lately, the desire to decrease the negative ramifications associated with the use of internal combustion engine powered transportation [7–9], and in particular the drive to decrease carbon emissions [10,11], has led to a significant resurgence in interest in electrified transportation. Of particular importance to enable electrified transportation is the availability of economically and technologically robust batteries. The Progression of Battery Technologies Used for EV Applications Many different kinds of batteries exist, and as new systems are developed to commercial maturity, they have been applied to the problem of electrified transportation. A Ragone plot of some of the more common battery technologies is shown in Figure1[12–14]. Energies 2019, 12, 1074; doi:10.3390/en12061074 www.mdpi.com/journal/energies Energies 2019, 12, x FOR PEER REVIEW 2 of 20 The Progression of Battery Technologies Used for EV Applications Many different kinds of batteries exist, and as new systems are developed to commercial maturity,Energies 2019 they, 12, 1074 have been applied to the problem of electrified transportation. A Ragone plot of some2 of 20 of the more common battery technologies is shown in Figure 1 [12–14]. Early EV applications used the rechargeable Lead-Acid battery developed in 1859 by Gaston PlantéEarly [1]. In EV 1899, applications Waldemar used Jungner the rechargeable introduced the Lead-Acid nickel-cadmium battery developed battery that in made 1859 significant by Gaston improvementsPlanté [1]. In 1899, in storage Waldemar capacity Jungner but introducedhad some drawbacks the nickel-cadmium including batterya voltage that suppression made significant issue thatimprovements occurs as the in storagebattery capacityaged, known but had as somea memo drawbacksry effect including[15]. Research a voltage continued suppression through issue the beginningthat occurs and as thelatter battery half of aged, the 20th known century as a but memory it was effect not until [15]. 1985 Research that the continued first lithium-ion through (Li- the ion)beginning batteries and were latter created. half of theIt took 20th centurya further but 6 years it was of not research until 1985 before that thethey first were lithium-ion commercialized (Li-ion) [15,16].batteries In were the created.meantime, It took EVs a using further ZEBRA 6 years batte of researchries and before Nickel-Metal they were Hydride commercialized batteries [ 15were,16]. developedIn the meantime, [17]. The EVs current using ZEBRA predominant batteries battery and Nickel-Metal energy storage Hydride technology batteries for were EVs developed is the Li-ion [17]. battery.The current predominant battery energy storage technology for EVs is the Li-ion battery. Figure 1. RagoneRagone plot plot of several of the battery technologies used in EVs [12]. [12]. Batteries are are fundamentally fundamentally a astorage storage medium medium made made up upof two of twoelectrodes electrodes in an in electrolyte. an electrolyte. This electrolyteThis electrolyte provides provides a medium a medium for the for exchange the exchange of ions ofwhich ions produces which produces the electricity the electricity [14]. Each [14 of]. theEach batteries of the batteries shown in shown Figure in 1 Figure has their1 has own their uniq ownue unique advantages advantages and disadvantages, and disadvantages, though though recent innovationsrecent innovations in Li-ion in batteries Li-ion batteries have propelled have propelled them to them become to become the market the market leader leaderfor use for in use most in handheldmost handheld and portable and portable electronics electronics as well as well as EVs. as EVs. This This is isprimarily primarily due due to to their their specific specific energy (Wh/kg), cycle cycle life life and and high high efficiency efficiency [14–18]. [14–18]. Th Theyey do do have have downsides downsides which which include include their their high high cost cost and the need for complex safety and monitoring systems [[14].14]. This paperpaper will will present present an overviewan overview of several of seve differentral different types of types Li-ion batteries,of Li-ion theirbatteries, advantages, their advantages,disadvantages, disadvantages, and opportunities and opportunities with Li-ion energy with Li-ion storage energy as it relates storage to as EVs. it relates It will concludeto EVs. It withwill concludea brief overview with a ofbrief ways overview to recycle of ways or reuse to batteriesrecycle or that reuse have batteries reached that their have end reached of life in their EVs asend well of lifeas discuss in EVs someas well additional as discuss research some additional opportunities. research opportunities. 2. The The Li-Ion Li-Ion Batteries Batteries In general, Li-ion batteries can can be characterized as as energy energy storage systems systems that that rely rely on insertion reactions from both electrodes where lithium ions act as the charge carrier [[18].18]. Given this broad definition,definition, there there are are several several different different cell cell chemistries chemistries that that make make up up the the Li-ion Li-ion battery battery family. family. Most Most Li- ionLi-ion batteries batteries use use a negative a negative electrode electrode [19] [19 principa] principallylly made made from from carbon carbon (e.g., (e.g., graphite) graphite) or or lithium titanate (Li Ti O ), with some novel materials under development, namely, Li metal and Li(Si) alloys. titanate (Li4Ti55O1212), with some novel materials under development, namely, Li metal and Li(Si) alloys. The electrolyte used varies based on the choice of electrode materials, but is typically composed of a mixture of lithium salts (e.g., LiPF ) and an organic solvent (e.g., diethyl carbonate) to allow for ion 6 transfer—these components will be discussed in more detail below. A separating membrane is used to allow lithium ions to pass between the electrodes while preventing an internal short circuit [20]. Energies 2019, 12, x FOR PEER REVIEW 3 of 20 mixture of lithium salts (e.g., LiPF6) and an organic solvent (e.g., diethyl carbonate) to allow for ion Energiestransfer—these2019, 12, 1074 components will be discussed in more detail below. A separating membrane is 3used of 20 to allow lithium ions to pass between the electrodes while preventing an internal short circuit [20]. This arrangement is shown conceptually in Figure 2, with the transport aspects of the battery when Thisoperating arrangement as an energy is shown source conceptually (i.e., a galvanic in Figure device)2, with illustrated—the the transport aspects electrons of the travel battery from when the operatingnegative electrode as an energy to the source positive (i.e., electrode a galvanic while device) simultaneously illustrated—the the electrons Li+ ions travel travel from from the negative electrode tothrough the positive the electrolyte electrode whileto the simultaneously positive electrode the Lito+ ionsmaintain travel electroneutrality. from the negative When electrode the throughsystem is the operated