Designing High-Voltage Cathode and Electrolyte Interphase (CEI) with In Situ Formation, Passivation and Self-Healing Mechanisms

Cobalt-free LiNi_0.5Mn_1.5O₄ (LNMO) spinel is a promising candidate for the next generation cathode material based on its high operating voltage (4.75 V_{vs Li}), low material cost, and excellent rate capability. However, recent studies reported that the most critical barrier for the commercialization of LNMO is electrolyte oxidation and concurrent parasitic reactions at electrode/electrolyte interfaces, which led to a poor cycle life of LNMO/graphite full-cells. To mitigate the issue, various approaches have been proposed such as coating LNMO, tuning chemical compositions of LNMO, coating graphite anodes, using electrolyte additives, and adopting functional binders for LNMO. However, there is no simple solution to resolve all the complex issues occurring in the high-voltage cells. For example, physical coating on electrode materials will eventually fail due to mechanical cracking of cathode particles due to its repeated volume changes during cycling. In addition, not only LNMO but carbon conductors (e.g., acetylene black) inside a cathode leads to unwanted electrolyte oxidation prompted by its high electronic conductivity and large surface area. Considering that such parasitic reactions originate from cathode-electrolyte interphase (CEI) and propagate to anode solid-electrolyte interphase (SEI), there is a dire need of multimodal strategies that can create synergistic effect on stabilizing the electrode-electrolyte interphases in cell level.<br/>To address these complex interfacial issues, we developed a strategy to innovate the LNMO CEI by endowing critical functionalities – (i) in-situ formation of CEI, (ii) self-healing CEI, (iii) passivation of all LNMO, carbon conductor, and Al-foil. After screening candidates of CEI elements on LNMO, titania was selected as the main ingredient of CEI based on its slow dissolution kinetic at the presence of hydrofluoric acid and its capability to form LiNi_0.5Mn_1.5-xTi_xO₄ (LNMTO) solid-solution phase. Since increasing amount of Ti-substitution (x) in LNMTO reduces bulk electrical conductivity and specific capacity, we designed and synthesized a core (LNMO) – shell (LNMTO) structured cathode powders. This core-shell cathode can produce Ti-enriched CEI layer in-situ through a partial dissolution of Mn and Ni into electrolyte during cycling. If the existing CEI was damaged by a crack, the freshly revealed cathode surface can be recovered by a new, local CEI via the same interfacial reaction (i.e., partial metal dissolution), which can be referred to the “self-healing” mechanism. At the same time, lithium polyacrylate binder was adopted to passivate all LNMO, carbon conductor, and Al-foil against oxidative decomposition reactions at CEI. The combination of inorganic CEI (i.e., Ti-enriched oxides) and organic CEI (i.e., lithium polyacrylate) mimics the physical distribution and functionality of the graphite SEI.<br/>This presentation will also focus on the effect of the multifunctional CEI on battery performances. To avoid a corrosion issue of coin-cell parts made with stainless steel, which critically degraded the full-cell performance at high-voltages, battery performances was examined by using pouch-type cells. The full-cells having the multifunctional CEI retained &gt; 90 % of its half coin-cell capacity at C/3-rate with much improved cycle life compared to baseline performance obtained from the conventional LNMO cathode. AC impedance spectroscopy combined with the distribution of relaxation times (DRT) technique revealed a suppression of cell interfacial resistance during cycling, suggesting the enhanced stability of the multifunctional CEI. Finally, microscopy and spectroscopy data obtained from cycle-aged CEI and graphite SEI will be corelated to the cell performance data to identify the CEI property-performance relationship. This unique multifunction CEI strategies will be applicable to various cathode materials for the next-generation Li-ion batteries.