Lithium cobalt silicate Li2CoSiO4 is a promising but very challenging high energy cathode material for lithium-ion battery. This work presents a systematic study of synthesis-structure-performance of Li2CoSiO4, in which carbon coating and P doping are incorporated in nano-particles synthesized by hydrothermal reactions and the collaboration among different mechanisms of modification results in controlled synthesis of polymorphs and advanced electrochemical performance. The carbon coated Li2CoSiO4 sample exhibits improved electrochemical performance with a reversible capacity of 112 mAh g−1 at room temperature. Furthermore, 10% P substituted Si in Li2CoSiO4/C demonstrates a single form in polymorphs with much higher discharge capacity of 144 mAh g−1. The analysis of cyclic voltammograms and ex situ XRD indicates Li2CoSiO4 bears electrochemical characters of stable structure and high discharge voltage plateau during cycling. First-principles calculations on P-doping Li2CoSiO4 models find the P-doping site to be a repulsively positive center that suppresses the asymmetrical stress of Li+ and Peierls distortion in the delithiated structures, therefore promoting the deintercalation in the electrochemical performance.