The electrochemical rate performance and capacity retention of the "layered-layered" lithium rich Li1.2Mn 0.525Ni0.175Co0.1O2 (Li-rich NMC) material are significantly improved by a nanometer layer coating of a lithium conducting solid electrolyte, lithium phosphorus oxynitride (LiPON). The LiPON layer is deposited on the Li-rich NMC particles by the RF-magnetron sputtering method. The presence of the LiPON layer provides interfacial stability under high current (rate) and voltage cycling conditions and thereby improves the capacity retention over cycle life compared to pristine or uncoated Li-rich NMC. Specifically, the LiPON coated Li-rich NMC composite electrode showed stable reversible capacities of >275 mAh g-1 when cycled to 4.9 V for more than 300 cycles, and showed at least threefold improvements in the rate performance compared to the uncoated electrode compositions. Increasing the LiPON layer thickness beyond a few nanometers leads to capacity fade due to increasing electronic resistance. Detailed microstructural and electrochemical impedance spectroscopy studies are undertaken to characterize and understand the role of LiPON in improving the interfacial stability and electrochemical activity at the interface. © 2013 The Royal Society of Chemistry.