Operando X-ray spectroscopy study on a high-voltage cathode and polymer-Li-conducting solid electrolyte interface for dendrite-free solid-state lithium metal batteries

Nickel-rich lithium nickel manganese cobalt oxide (LiNi_0.6Mn_0.2C_0.2O₂, NMC 622) cathodes commonly encounter capacity loss in lithium metal batteries at high voltages (>4.2 V) due to excessive parasitic reactions and structural degradation in carbonate-based liquid electrolytes (LEs). Substituting LEs with solid polymer electrolytes faces challenges such as low lithium-ion transference number (t_Li⁺), ionic conductivity (σ_ion), and mechanical strength (MS) at room temperature. Addressing these limitations, a nano Li_1.6Al_0.5Ge_1.5P_2.9Si_0.1O₁₂-rich fused conductive network-based hybrid solid polymer electrolyte (IRHSPE-50) is developed, exhibiting exceptional t_Li⁺ of 0.75, σ_ion of 1.42 mS cm⁻¹ and MS of 13.3 Mpa at room temperature (30 °C). The enhanced performance is attributed to optimal LAGPS content, facilitating fast Li⁺ movement through a conductive network. Utilizing IRHSPE-50, solid-state lithium metal batteries (SSLMBs) with NMC 622 cathodes achieve a capacity of 179.44 mAh g⁻¹ at 0.2C under 30 °C with 79.9 % capacity retention over 250 cycles. In-situ synchrotron X-ray near-edge absorption spectroscopy (SXANES) and X-ray diffraction (SXRD) studies reveal cobalt irreversibility during delithiation, maintaining structural integrity with minimal volume change (2 %) and no additional phase formation during cycling. The IRHSPE-50 membrane establishes a stable interface with the NMC 622 cathode, creating a thin and uniform cathode-electrolyte interphase layer that effectively suppresses interfacial reactions. The formation of an ion-conducting lithium fluoride layer and an outer organic layer on the Li surface enables uniform and dendrite-free Li⁺ transport with a critical current density of 2 mA cm⁻², preventing active Li loss and mitigating NMC 622/IRHSPE-50 degradation. Facile development and a fundamental understanding of IRHSPE-50, interface chemistry, and degradation mechanisms are poised to accelerate the advancement of high-performance SSLMBs.