Sodium-ion batteries （SIBs） are poised to revolutionize the global energy storage landscape， offering a cost-effective and resource-abundant alternative to lithium-ion batteries. Within this emerging ecosystem， hard carbon （HC） has secured its position as the premier anode material， prized for its structural stability and low operating voltage. Yet， the transition from academic discovery to GWh-scale manufacturing faces formidable hurdles. The commercial viability of HC is currently compromised by a low initial coulombic efficiency （ICE）， which imposes a severe economic penalty by forcing cathode oversizing， and by significant engineering gaps in precursor scalability and full-cell integration. This review aims to bridge the critical gap between the fundamental understanding of sodium storage mechanisms and the practical realities of battery engineering. First， we begin by consolidating the latest consensus on the adsorption-intercalation-pore filling mechanism， establishing a solid theoretical framework for rational design. Unlike traditional reviews that focus solely on capacity metrics， we critically evaluate the industrial feasibility of advanced synthesis strategies， highlighting the hidden trade-offs between heteroatom doping and efficiency， and addressing the scalability challenges of biomass precursors， such as ash content and consistency. Second， we provide a comprehensive analysis of the ICE challenge， linking it directly to cell cost， and evaluate mitigation strategies including advanced electrolytes and pre-sodiation. Finally， the review addresses specific full-cell failure modes， such as gassing and low volumetric density， offering a pragmatic roadmap for developing robust， commercially viable hard carbon anodes.

Contents

1 Introduction

2 Understanding of Na-ion storage mechanism in HC

2.1 Fundamentals of hard carbon

2.2 Sodium ions storage mechanisms

2.3 Advanced characterization techniques

3 Advanced synthesis and structural engineering

3.1 Biomass-derived hard carbons

3.2 Heteroatom doping

3.3 Morphological and pore structure engineering

4 How to solve the low ICE

4.1 Understanding the root causes of low ICE

4.2 Electrolyte engineering （mitigation）

4.3 Surface engineering and coatings （mitigation）

4.4 Pre-sodiation （compensation）

5 Full-cell development： bridging the gap to commercialization

5.1 Sodium inventory problem

5.2 Pouch cell and cylindrical cell prototyping

5.3 Practical failure modes

6 Challenges and future perspectives

7 Conclusions