Nanoscale, LiNiPO4-core and carbon-shell high voltage (>5 V) LiNiPO4@C cathode materials have been synthesized using a microwave & solvothermal methodology using different solvents, ethylene glycol, isopropanol, isobutanol and water as the reaction medium. The effects of these solvents on the crystal-quality, morphology and electrochemical qualification of the produced materials have been evaluated in terms of the heating efficiency in the microwave field by using various opto-analytical techniques and electrochemical measurements. The heating characteristics in terms of the absorption of the energy as part of the microwave-material interaction phenomenon is also discussed. X-ray diffraction analyses demonstrate that it is possible to synthesize substantially pure LiNiPO4 crystal using isopropanol as the reaction medium. High-resolution transmission electron microscopy analysis reveals that this combined methodology can provide core-shell morphology with a 5–6 nm coating thickness. The LiNiPO4@C cathode material produced in an isopropanol environment exhibits the best electrochemical properties, achieving a discharge capacity of 157 mAh g−1 at a 0.l C-rate, and shows almost 81% capacity retention at the end of the 80th cycle. Thus, this paper offers a perspective for solving the difficulties encountered in modifying a high voltage LiNiPO4 cathode, especially the deficiency in terms of cycle life behavior, and the further benefits are highlighted.