INTRODUCTION
Rechargeable batteries play a key role in
liberating human production and life
from dependence on fossil fuels and reducing greenhouse gas
emissions.[1-3] Among them, lithium-ion batteries (LIBs) are
generally composed of metal-containing active materials (Li, Co, Ni,
etc.), flammable organic electrolytes
and non-degradable polymer membranes (polyolefin).[4-6] However, the
growing demand for LIBs is pushing up the price of raw materials and the
existing battery materials cannot satisfy the energy demand.
Accordingly, it is urgent to develop renewable, sustainable and green
battery materials to realize sustainable development.[7-9] In this
regard, natural biomaterials are
increasingly attracting interest in green batteries due to the numerous
advantages of biopolymers such as broad-resource, low-cost and
environmental protection.[10, 11]
In the composition of LIBs,
separator directly determines the
electrochemical performance and the safety of the battery.[12-15]
Although it does not directly participate in the electrochemical
reaction of LIBs, its physical properties and structure play a key role
in LIBs. Currently, widely used separators mainly include microporous
polyolefin membranes formed by insulating polymer materials. However,
due to its characteristics of large polarity difference with electrolyte
and low thermal melting temperature, PP separators have some
shortcomings such as poor infiltrative properties and poor thermal
stability, which leads to a large safety hazard of LIBs.[16, 17]
Therefore, it is urgent to develop next-generation separators to replace
the existing polyolefin separator.
Cellulose and its derivatives are good alternatives to polyolefin-based
materials because they are abundant in reserves, renewable, easy to
access and low cost.[18-20] Till now, many researchers have reported
novel cellulose-based membranes. For example, Du et al.[21] prepared
a cellulose based gel membrane using epichlorohydrin (ECH) as a
crosslinking agent. The ionic conductivity of the activated cellulose
membrane with 5% ECH achieves 6.34 mS cm-1. What’s
more, the assembled NCM523/Li battery can reach 145 mAh
g-1 at 0.2 C, and the capacity retention rate is as
high as 90% after 50 cycles. Liu et al.[19] prepared
lignin-containing cellulose nanofibers (LCNFs) membranes from the
unbleached pulp by a facile method. The LCNF porous membrane shows a
high electrolyte uptake of 276% and high ionic conductivity of 1.86 mS
cm-1. With the higher requirements for battery safety,
the composite membrane combining ceramic particles and polymer has
attracted more attention from researchers. Thereby, nano-inorganic
particles such as Al2O3,
SiO2 and TiO2 can significantly improve
electrolyte wettability, thermal stability and ionic conductivity of
polymer.[22-25] Recently, Cui et al.[26] prepared a
polyvinylidene fluoride (PVDF)/cellulose acetate (CA) based membrane by
using non-solvent induced phase separation (NIPS) method. Meanwhile,
Al(OH)3 particles were used as added doping agents. The
PVDF-CA/Al(OH)3 separator has high porosity (68.6%),
high electrolyte absorption rate (403.9%), excellent dimensional
stability (4.6% thermal shrinkage at 160°C) and high ionic conductivity
(2.85 mS cm-1), endowing LIBs with good safety
performance and cycling performance. However, batteries still suffer
from a lack of cycle durability because LiPF6 in liquid
electrolytes can form hydrofluoric acid (HF) to damage the
electrolyte/elctrodes interfaces.[27, 28]
Herein, nano-CaCO3 is introduced into cellulose-based
electrospinning membranes to reduce the crystallinity of polymer
membrane, promote ion migration and neutralize acidic products. The
morphology, thermal stability, mechanical properties and electrochemical
properties of modified membranes are analyzed and explored in depth.
Such inorganic particles with extremely high hydrophilicity and
excellent surface properties can significantly improve the ionic
conductivity of membrane. In addition, the rigidity and heat resistance
of CaCO3 guarantee the mechanical strength and thermal
stability of membrane. Consequently, the composite membrane is superior
to polypropylene (PP) separator in electrolyte wettability and thermal
stability, leading to the outstanding performance of
LiFePO4/Li cells. We believe that
cellulose/CaCO3 composite membrane with the above
advantages is a promising candidate for practical applications in
advanced LIBs.