All-solid-state lithium-ion batteries (ASSLIBs) are considered the most promising option for next-generation high-energy and safe batteries. Herein, a practical all-solid-state battery, with a Li- and Mn-rich layered oxide (LMRO) as the cathode and Li6PS5Cl as the electrolyte, is demonstrated for the first time. The battery delivers the most exceptional performance by far in terms of ultrahigh capacity of 244.5 mA h g-1 and unprecedented cycling stability with an 83% capacity retention after 1000 cycles. We discover that the Li6PS5Cl can be reversibly oxidized and reduced within the voltage range 2.0-4.8 V, which is beneficial to the ionic conduction during long-term cycling of ASSLIBs. Moreover, the electronic and ionic conductivities of LMROs are increased by 4 orders of magnitude via precisely tailoring the composition and structure. In addition, the typical dissolution of transition metal, oxygen release, and phase transformation of LMROs in liquid batteries are substantially eliminated in ASSLIBs.