Nov_Dec_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 7 1 7 computers [5] . These batteries are light, compact, exhibit good charge reten- tion, and have no memory effect. In the 1990s, LiFePO 4 (LFP) was discovered to feature superior thermal stability and environmental impact compared to LiCoO 2 . The initial challenge of the LFP chemistry was its low electrical conduc- tivity and mass transfer resistance, but these limitations were overcome by car- bon coating (C-LFP) and particle size re- duction [6] . Such improvements of the C-LFPperformance strengthened its po- sition as the optimum cathode choice when considering characteristics such as specific power (W/kg), thermal sta- bility, cyclability, and safety. The vast majority of commer- cially available lithium-based batter- ies belong to the LIB category and use carbon as anode, LiPF 6 in organic sol- vent as electrolyte, and cathode mate- rials such as mixed oxides (e.g., NMC: Li(Ni/Mn/Co)O 2 ; NCA: Li(Ni/Co/Al)O 2 and LFP) for various applications. The ad- vantages of LFP for large batteries comes from the potential low cost that can be achieved by using widely avail- able precursors such as iron and phos- phorus. This is in contrast to expensive cathode chemistries such as NMC and NCA that contain cobalt with concerns about availability, toxicity, and rising cost (Fig. 3). EV AND PHEV APPLICATIONS Requirements for LIBs for EVs and PHEVs include a lifespan of 8-15 years, operating temperature range between -40 o and 60 o C, vibration resistance, af- fordable cost, and safety [3] . Despite im- provements in LIB performance, today’s C-LFP cathode materials struggle to be- come the first choice for automotive batteries. This is due to the lower ener- gy density of LFP relative to metal oxide alternatives along with high LFP mate- rial costs, owing to energy intensive and Fig. 2 — Various battery chemistries and their corresponding energy density (Wh/L) and specific energy (Wh/kg). Source: electropaedia.com . S tringent Corporate Average Fuel Economy (CAFE) standards in North America require 54.5 miles per gallon (~4.5 L/100 km) fuel econo- my for passenger vehicles by 2025. In- dustry has responded to this challenge and expanded their R&D activities. They are predominantly focused on vehicle weight reduction as well as internal com- bustion engine (ICE) power efficiencies [1] . These efforts have already made an im- pact. However, achieving appreciable and sustainable levels of decarbonization requires that consumers broadly adopt electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) (Fig. 1). Battery packs and cathode chem- istry are key components of the EV drive systems that affect capital cost as well as performance. Battery pack development is crucial for success- ful implementation of EVs [2] . A number of battery chemistries have been de- veloped over the last decade (Fig. 2). Lithium-ion battery (LIB) compositions have attracted the most interest due to their higher energy density (Wh/kg) compared with other commercial bat- teries, and vary from 120-250 Wh/kg at the cell level. They weigh less than tra- ditional lead-acid batteries (~40 Wh/ kg) used for starting-lighting-ignition (SLI), and nickel-metal hydride (NiMH) (~90Wh/kg) batteries that are used to power almost all current hybrid electric vehicles (HEVs) [3,4] . Discovered in the 1980s, LiCoO 2 (LCO) became a key cathode material in LIBs for portable electronics. Sony commercialized this chemistry in the 90s and it has become the internation- al standard for portable electronic de- vices such as cellphones and laptop Fig. 1 — Contemporary hybrid and electric vehicles [8] .