IEEE Electrification - September 2020 - 64

Approaches: Current
Research Directions

Researchers have
reported that a
higher operating
temperature
improves the
sluggish kinetics of
the battery.

Cell Design
Anode

As identified in the previous section,
Li plating on the graphite anode is
the primary limiting factor for the
charge rate capability. Therefore, the
search for alternative anode materials has drawn a lot of research attention recently. The most well-known
candidates are silicon, Li metal, and
Li4Ti5O12 (LTO). LTO has excellent charging rate capability
and cycle life; however, LTO cells exhibit poor energy
density. The cost of titanite also prevents it from widespread usage.
Lithiated silicon (Li15S4) has a potential of 50 mV versus
Li/Li+, which makes it more resilient to Li plating under
high charging current. The 3,579 mAh/g theoretical Li storage capacity of silicon is also significantly higher than
graphite (372 mAh/g). However, silicon anodes suffer from
severe volume changes during Li-ion intercalation and
deintercalation.
Li metal, due to its high specific capacity (3,681 mAh/g)
and low electrochemical potential (-3.04 V), possesses
excellent energy density. Different from the other hosting
anode materials like graphite, the Li metal is a hostless
conversion anode. The Li ions are plated onto the Li metal
anode during charging and are stripped from the anode
during discharge. However, the high reactivity of Li metal
results in parasitic reactions with electrolytes. This results
in low cycling stability and safety issues. For the adoption
of Li metal-based Li-ion batteries, the invention of stable,
conductive, solid-state electrolytes rather than volatile
organic liquid electrolyte is required.

TABLE 1. The capacity loss rate of electrodes
with various thickness.

Cathode
Anode
Thickness Thickness
(µm)
(µm)

Capacity Loss
Rate After 52
Full Cycles
at 1-C Charge,
C/3 Discharge

Capacity Loss
Rate After 52
Full Cycles at
1.5-C Charge,
C/3 Discharge

6.25%

5.3%

25.6%

101

127

41.7%

129

154

43.1%

154

182

56.4%

Data Source: Gallagher et al.

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2020

Electrolyte

Many researchers suggest that by
improving the Li-ion mass transport in
the electrolyte, the Li-ion depletion
near the electrode can be mitigated,
thus achieving reduced Li plating during fast charging. Du et al. compared
lithium bis(fluorosulfonyl)imide (LiFSI)
salt with conventional LiPF6 salt in a
Li-ion cell experimental study. LiFSI has
a 13% -higher ionic conductivity and a
29.5% higher Li-ion t-ransference number. Both electrolytes were tested with
Ni0.8Mn0.1Co0.1O2 (NMC811) cathodes
and graphite anodes. The cells were charged at a constant
current of 5 C until the voltage reached the cutoff voltage
of 4.2 V, and then the voltage was held at 4.2 V until the
total charging time reached 12 min. The sample with LiFSI
electrolyte scored 13% more initial charge capacity. After
500 cycles, the higher ionic conductive sample (with LiFSI)
retained 84% of the initial capacity, while the lower ionic
conductive cell (with LiPF6) retained only 77% of its initial
capacity. Other approaches, like using low-viscosity cosolvent, high concentrated electrolytes, and large polyanions,
have been reported in the literature to improve the ionic
conductivity, Li-ion transference number, and stability of
the electrolyte.

Charging Profile
As battery manufacturers and researchers continue
to push the limits of cell power density, energy density,
and cycle life through material improvement, significant
research is also being conducted focusing on improving these factors by optimizing the operation of the
cell in terms of temperature and current control. Many
researchers have tried to manipulate the current during charging and create a dynamic charging profile to
attenuate the negative effects of the high charging
rate. Sebastian et al. mapped the dc internal resistance
(DCIR) of a 3.2-Ah 18650 NCA(LiNi0.5Co0.15Al0.05O2) cell
across the full state of charge (SoC) and discovered two
high DCIR regions of the cell in the 0-40% SoC and
80-100% SoC ranges. In the 40-80% SoC range, the DCIR
was substantially lower. Based on the DCIR data, a DCIRbased fast-charging profile has been developed by applying a 2-C charging rate in the low DCIR region. The results
showed a 44% cycle life improvement over standard constant current/constant voltage charging.
Chu et al. used an electrochemical model to predict
localized anode potentials and adjust the charging current
rate accordingly to avoid Li plating. However, the computational requirement of the electrochemical model is
intensive. To ensure accuracy, the voltage and current of
each cell are required to be monitored in real time. Additional monitoring circuits need to be added into the pack
and BMS design for practical battery pack operation.

IEEE Electrification - September 2020

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