High-sodium brines, characteristic of many salar-hosted, geothermal, and oilfield lithium resources, impose fundamental limitations on conventional adsorption and membrane-based separation processes, as well as on electrodialysis (ED) and bipolar electrodialysis (BMED) systems. This work evaluates the mechanisms by which elevated sodium concentrations can reduce separation efficiency, increase operating costs, and constrain process design across direct lithium extraction (DLE) flow sheets. Elevated Na/Li ratios are also economically significant because they lower lithium yield per unit of brine processed, increase both energy and reagent demand, and complicate downstream purification, all of which materially worsen the cost of producing each tonne of high purity lithium product. In electrodialysis, sodium exerts a dominant influence on ion transport behaviour because its concentration is typically far greater than that of lithium. Migration through a cation-exchange membrane becomes competitive, as Na⁺, with its different charge density, is preferentially carried by the electric current. This effect is compounded by the limited ion-specific permselectivity of standard membranes, which are highly effective at charge discrimination but exhibit only weak differentiation between similar monovalent cations such as Li⁺ and Na⁺. Consequently, lithium recovery efficiency declines, particularly in brines with high Na/Li ratios, where sodium effectively dictates flux rates. Although high ionic strength can initially reduce solution resistance, overall energy consumption per unit of lithium recovered increases. This is driven by inefficient current utilisation, where much of the energy is spent transporting sodium towards the membrane interface rather than selectively recovering lithium. Scenarios with high Na/Li ratios therefore translate into greater electricity consumption, lower throughput, and the need for additional treatment stages to achieve a saleable product. The challenges are further amplified in BMED systems, where high concentrations of sodium salts favour NaCl splitting and NaOH co-production rather than the desired generation of lithium hydroxide. This leads to mixed NaOH/LiOH streams, dilution of product value, and increased downstream purification complexity. Current efficiency losses are significant, as electrical input is consumed by non-target salt-splitting reactions rather than lithium-specific product formation. In economic terms, this raises the cost of separation, adds polishing and crystallisation requirements, and weakens the commercial advantage of BMED when Na/Li ratios are high. Beyond ED and BMED, high sodium also impacts upstream and parallel membrane processes. Nanofiltration exhibits poor rejection of monovalent ions, limiting its effectiveness for Li/Na fractionation, while reverse osmosis performance is compromised by elevated osmotic pressures. While sodium itself is not a scaling species, its presence correlates with high total dissolved solids and the co-occurrence of divalent ions such as Ca2⁺, Mg2⁺, and SO₄2⁻, which contribute to membrane fouling, scaling, and increased maintenance requirements. These effects further add to life cycle costs and can reduce plant availability. In hybrid systems, high sodium can suppress lithium adsorption kinetics and compete for active sites, depending on sorbent selectivity. These limitations necessitate process adaptation. Viable strategies include the deployment of lithium-selective sorbents, the development of monovalent-selective or lithium-specific media, and the adoption of hybrid flow sheets where ED or BMED is applied for bulk salt management rather than primary lithium recovery. Pre-treatment to reduce competing ions and optimise brine chemistry is critical to maintaining system performance and improving economic viability. Actual cases of sodium’s impact on projects are usually framed as brine-chemistry or scale-up problems rather than as explicit ‘Na/Li ratio failures’. Public sources show that high-salinity, sodium 33 Critical Minerals Conference 2026 | Brisbane, Australia | 21–23 September 2026 rich brines make DLE harder to commercialise, push up costs, and delay ramp-up, while project documents and technical reviews emphasise the Na/Li ratio as a key performance metric. Global data were analysed across 67 research prospects, spanning nearly three orders of magnitude in Na/Li mass ratio, ranging from ~24 (Groß Schönebeck, Germany — a Rotliegend-hosted geothermal brine) to ~16 647 (Anadarko Basin, USA — a halite-dissolution oilfield brine). The findings indicate that geological provenance (lithology category) and basin type (enrichment/evaporation category) are the two main factors determining which domains are dominated by sodium interference and structurally disadvantaged for recovery. Caucharí-Olaroz, Argentina, is a useful benchmark for cases where Na/Li is not itself the limiting factor. The operation initially ran below nameplate capacity before moving closer to full capacity, illustrating how brine projects often face commissioning and ramp-up challenges even when the chemistry is workable. Lake Resources’ Kachi project in Argentina was delayed by six years, with cost expectations rising sharply, consistent with the broader challenge of making sodium-rich brines economic at scale. Salar de Uyuni’s development in Bolivia has been described as a very large but chemically challenging salar, where DLE was intended to manage brine-chemistry complexity. While several lithium brine projects have shown that sodium-rich chemistry can undermine project economics, in practice the issue is usually not sodium alone but the combined effects of difficult brine chemistry, lower lithium selectivity, fouling risk, and added purification costs, which together raise operating expenditure and slow ramp-up. Overall, high sodium shifts the function of lithium processing flow sheets from straightforward selective separation technology to supporting unit operations focused on acid/base generation or salinity control. Recognition of the economic and technical penalties associated with high Na/Li ratios is essential for realistic technology selection, process optimisation, and economic evaluation of next generation lithium brine developments.