Lithium-ion batteries (LIBs) have transformed modern electronics and rapidly advancing electric vehicles (EVs) due to high energy, power, cycle-life, and acceptable safety. However, the comprehensive commercialization of EVs necessitates the invention of LIBs with much enhanced and stable electrochemical performances, including higher energy/power density, cycle-life, and operation safety but at a lower cost. An unprotected lithium-ion battery (LIB) cell cathode using lithium metal anode and organic carbonate liquid electrolyte undergoes significant structural damage during the cycling (Li+ intercalation/ deintercalation) process. Also, a bare cathode in contact with a liquid electrolyte forms a resistive cathode electrolyte interface (CEI) layer. Both the cathode structure damage and CEI lead to rapid capacity fade.
Different strategies have been used to mitigate the degradation of LIB electrodes, including designing electrolytes to enhance SEI/CEI formation, cycle stability, interface engineering with protective coatings to prevent the breakdown of active material particles during cycling, composition control of the electrode particles, synthetic optimization to control particle morphology, the use of composites made from conductive scaffolds and active materials and designing new electrode architectures to overcome volume changes and enhance transport properties. Cathode surface modification has been used to reduce CEI formation and structural damage, improving capacity retention, cycle life, energy density, power density, and safety of a LIB.
Recently, the coating of the cathode with an intermediate layer (IL), which is transparent to Li+ conduction but impermeable to electrolyte solvent, has been developed to minimize CEI formation and structural damage. IL based on Li+ insulating ceramics, such as aluminum oxide (Al2O3), tin oxide (SnO2), and magnesium oxide (MgO), has been developed but to limited success in mitigating the above cathode degradation. The limited success of Li+ insulating coating relates to the limited thickness of coating because the resistance of the coating layer increases with the thickness of IL. To overcome the challenges associated with Li+ insulating IL, recently, Li+ conducting IL (solid-state ceramic electrolytes) has been explored. Some of the most studied ceramic solid electrolytes include lithium niobate (LNO), lithium lanthanum zirconium oxide (LLZO), lithium aluminum titanium phosphate (LATP), etc. Though LNO (σ = 10-5 mS.cm-1) and LLZO (σ = 10-4 mS.cm-1), LATP (σ = 10-4 mS.cm-1) are better Li+ conductor compared to complete Li+ insulating ILs (Al2O3, MgO, SnO2) but still not adequate for high-performance LIB. Lithium aluminum germanium phosphate (LAGP- Li¬1.5Al0.5Ge1.5(PO4)3 ) has one order higher Li+ conduction (σ = 10-3 S.cm-1) compared to LATP.
In this dissertation, firstly, we introduce a LAGP sol-gel low-cost direct wet mixing coating method to enhance the electrochemical performance and structural stability/integrity of Lithium Cobalt Oxide ( LCO – LiCoO2) cathode material in LIB. Secondly, we optimized the LAGP coating on LCO to make the most out of the protective ceramic solid electrolyte coating layer on LCO to have the best possible performance, electrochemically, structurally, thermally, and safely. And finally, we introduced and implemented this optimized LAGP coated cathode into all solid-state lithium-ion batteries (ASSLIB) together with lithium metal anode to improve the performance and safety.