In the work, new lithium-conducting solid electrolytes based on lithium zirconate are synthesized. They are obtained by doping Li8ZrO6 phase with isostructural Li7TaO6. It is shown that in the Li8– xZr1– xTaxO6 system, a series of solid solutions х = 0−0.5 based on Li8ZrO6 form. The conductivity of synthesized Li8 ‒ xZr1 – xTaxO6 solid solutions increases by 1–2 orders of magnitude as compared with undoped zirconate Li8ZrO6 due to the formation of lithium vacancies in the tetra- and octahedral layers of the structure. All-solid-phase electrochemical cells with Li7.85Zr0.85Ta0.15O6 electrolyte, 0.75Li2SnMo3O12 ∙ 0.25B2O3 glass-ceramic anode, and 0.2Li2O · 0.2LiF · 0.45V2O5 · 0.25B2O3 cathode are electrochemically tested. It is shown that the resistance of 0.75Li2SnMo3O12 · 0.25B2O3|Li7.85Zr0.85Ta0.15O6|0.2Li2O · 0.2LiF · 0.45V2O5 · 0.25B2O3 cell decreases after the charge—discharge cycling.