The potential energy landscape of lithium borate glass of composition Li₃B₇O₁₂ has been investigated by the charge attachment induced transport (CAIT) technique. Here, native lithium ions have been replaced by foreign alkali ions, M⁺ = K⁺, Rb⁺, Cs⁺. All experiments exhibit a pronounced decrease of native ion diffusion coefficients over more than 4 orders of magnitude with decreasing local population of Li⁺. The energy landscape is modelled by a site energy distribution (SED) with a concentration dependent Fermi energy of the native Li⁺ ions. The width of the populated part of the SED is found to be 250 meV (FWHM). The conclusion is made possible by a combination of a macroscopic ion replacement experiment with a Nernst–Planck–Poisson modelling of concentration depth profiles measured by secondary ion mass spectrometry (SIMS). Possible generalizations of macroscopic transport theory to match an Onsager ansatz are discussed.