More than a grain of salt: technological substitution and the instability of critical mineral policy

Critical mineral policy rests on an underexamined premise: that the importance of specific minerals is stable rather than contingent on evolving technological systems. This paper makes two operational contributions. First, it specifies technological substitution as a monitorable seven-stage sequence with observable indicators. Second, it defines the criticality lag: the jurisdiction-specific interval between commercially meaningful market entry of a substituting technology (t₁) and material revision of classifications or policy commitments (t₂). The framework is applied prospectively to sodium-ion batteries, an unfolding development with the potential to reconfigure lithium demand across stationary storage, short-range mobility, and hybrid battery systems. Using a structured, case-oriented evidence synthesis, process tracing, and three bounding scenarios, the analysis distinguishes demonstrated performance from realised deployment, contracted capacity, corporate announcements, and projections. The evidence does not establish that sodium-ion will displace lithium-ion across the battery market: energy density, hard-carbon supply, manufacturing concentration, and uncertain cost parity remain substantial constraints. It does show that sodium-ion has moved beyond a purely theoretical alternative and has entered commercially relevant niches. The consequential policy question is therefore not whether lithium ceases to be critical, but whether criticality frameworks can detect material reordering before classifications and investment commitments become misaligned. The paper concludes that criticality assessment should shift from fixed classification toward time-bounded, technology-conditional monitoring, and specifies the indicators through which this can be operationalised.