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Layered-oxide $\mathrm{LiNi_xMn_yCo_{1-x-y}O_2}$ (NMC) positive electrodes
with high Nickel content, deliver high voltages and energy densities. However,
a high nickel content, e.g., $x$ = 0.8 (NMC 811), can lead to high surface
reactivity, which can trigger thermal runaway and gas generation. While claimed
safer, all-solid-state batteries still suffer from high interfacial resistance.
Here, we investigate niobate and tantalate coating materials, which can
mitigate the interfacial reactivities in Li-ion and all-solid-state batteries.
First-principles calculations reveal the multiphasic nature of Li-Nb-O and
Li-Ta-O coatings, containing mixtures of $\mathrm{LiNbO_3}$ and
$\mathrm{Li_3NbO_4}$, or of $\mathrm{LiTaO_3}$ and $\mathrm{Li_3TaO_4}$. The
concurrence of several phases in Li-Nb-O or Li-Ta-O modulates the type of
stable native defects in these coatings. Li-Nb-O and Li-Ta-O coating materials
can form favorably lithium vacancies $\mathrm{Vac^{'}_{Li}}$ and antisite
defects $\mathrm{Nb^{\bullet \bullet \bullet \bullet}_{Li}}$
($\mathrm{Ta^{\bullet \bullet \bullet \bullet}_{Li}}$) combined into
charge-neutral defect complexes. Even in defective crystalline
$\mathrm{LiNbO_3}$ (or $\mathrm{LiTaO_3}$), we reveal poor Li-ion conduction
properties. In contrast, $\mathrm{Li_3NbO_4}$ and $\mathrm{Li_3TaO_4}$ that are
introduced by high-temperature calcinations can provide adequate Li-ion
transport in these coatings. Our in-depth investigation of the
structure-property relationships in the important Li-Nb-O and Li-Ta-O coating
materials helps to develop more suitable calcination protocols to maximize the
functional properties of these niobates and tantalates.