Tunnel Intergrowth Li<i>_x</i>MnO₂ Nanosheet Arrays as 3D Cathode for High‐Performance All‐Solid‐State Thin Film Lithium Microbatteries

All-solid-state thin film lithium batteries (TFBs) are proposed as the ideal power sources for microelectronic devices. However, the high-temperature (>500 °C) annealing process of cathode films, such as LiCoO₂ and LiMn₂ O_4, restricts the on-chip integration and potential applications of TFBs. Herein, tunnel structured Li_x MnO₂ nanosheet arrays are fabricated as 3D cathode for TFBs by a facile electrolyte Li⁺ ion infusion method at very low temperature of 180 °C. Featuring an interesting tunnel intergrowth structure consisting of alternating 1 × 3 and 1 × 2 tunnels, the Li_x MnO₂ cathode shows high specific capacity with good structural stability between 2.0 and 4.3 V (vs. Li⁺ /Li). By utilizing the 3D Li_x MnO₂ cathode, all-solid-state Li_x MnO₂ /LiPON/Li TFB (3DLMO-TFB) has been successfully constructed with prominent advantages of greatly enriched cathode/electrolyte interface and shortened Li⁺ diffusion length in the 3D structure. Consequently, the 3DLMO-TFB device exhibits large specific capacity (185 mAh g^-1 at 50 mA g^-1 ), good rate performance, and excellent cycle performance (81.3% capacity retention after 1000 cycles), outperforming the TFBs using spinel LiMn₂ O₄ thin film cathodes fabricated at high temperature. Importantly, the low-temperature preparation of high-performance cathode film enables the fabrication of TFBs on various rigid and flexible substrates, which could greatly expand their potential applications in microelectronics.